1,721,023 research outputs found
From nonequilibrium Green’s functions to quantum master equations for the density matrix and out-of-time-order correlators: Steady-state and adiabatic dynamics
No full textWe consider a finite quantum system under slow driving and weakly coupled to thermal reservoirs at different temperatures. We present a systematic derivation of the quantum master equation for the density matrix and the out-of-time-order correlators. We start from the microscopic Hamiltonian and we formulate the equations ruling the dynamics of these quantities by recourse to the Schwinger-Keldysh nonequilibrium Green’s function formalism, performing a perturbative expansion in the coupling between the system and the reservoirs. We focus on the adiabatic dynamics, which corresponds to considering the linear response in the ratio between the relaxation time due to the system-reservoir coupling and the time scale associated to the driving. We calculate the particle and energy fluxes. We illustrate the formalism in the case of a qutrit coupled to bosonic reservoirs and of a pair of interacting quantum dots attached to fermionic reservoirs, also discussing the relevance of coherent effects.Fil: Bhandari, Bibek. Consiglio Nazionale delle Ricerche; Italia. Rochester Institute of Technology; Estados UnidosFil: Fazio, Rosario. Università degli Studi di Napoli Federico II; Italia. Centro Internazionale di Fisica Teorica; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Taddei, Fabio. Consiglio Nazionale delle Ricerche; ItaliaFil: Arrachea, Liliana del Carmen. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias Físicas. - Universidad Nacional de San Martín. Instituto de Ciencias Físicas; Argentin
Thermodynamic uncertainty relations for systems with broken time reversal symmetry: the case of superconducting hybrid systems
Full text linkWe derive bounds to the thermodynamic uncertainty relations in the
linear-response regime for steady-state transport in two-terminal systems when
time reversal symmetry is broken. We find that such bounds are different for
charge and heat currents and depend on the details of the system, through the
Onsager coefficients, and on the ratio between applied voltage and temperature
difference. As a function of such a ratio, the bounds can take any positive
values. The bounds are then calculated for a hybrid coherent superconducting
system using the scattering approach, and the concrete case of an Andreev
interferometer is explored. Interestingly, we find that the bound on the charge
current is always smaller than 2 when the system operates as a heat engine,
while the bound on the heat current is always larger than 2 when the system
operates as a refrigerator.Comment: 8 pages, 5 figure
Nonlocal topological valley transport at large valley Hall angles
Full text linkBerry curvature hot spots in two-dimensional materials with broken inversion symmetry are responsible for the existence of transverse valley currents, which give rise to giant nonlocal dc voltages. Recent experiments in high-quality gapped graphene have highlighted a saturation of the nonlocal resistance as a function of the longitudinal charge resistivity ρc,xx , when the system is driven deep into the insulating phase. The origin of this saturation is, to date, unclear. In this work we show that this behavior is fully compatible with bulk topological
transport in the regime of large valley Hall angles (VHAs). We demonstrate that, for a fixed value of the valley
diffusion length, the dependence of the nonlocal resistance on ρc,xx weakens for increasing VHAs, transitioning
from the standard ρ3
c,xx power law to a result that is independent of ρc,xx
Andreev levels spectroscopy of topological three-terminal junctions
No full textWe calculate the differential conductance at a probe inserted in the weak link of a topological Josephson junction, consisting of a semiconducting nanowire deposited on top of two separated superconductors. Our aim is to understand how the peculiar features in the spectrum of Andreev bound states, arising due to the presence of Majorana bound states at the ends of the two topological superconducting wires defining the junction, can be determined through a measurement of the differential conductance. We find that when the probe allows a single propagating mode, the differential conductance presents a dip at zero voltage of zero conductance close to the position where the spectrum exhibits the topologically protected crossing. This can be viewed as a signature of the presence of Majorana states, which does not require fermion parity conservation and is robust against parameters' changes, as well as disorder. On the contrary, when the probe allows two or more propagating modes the differential conductance resembles the spectrum of Andreev bound states. This has been established making use of both numerical and analytical methods
ΔT-noise in multiterminal hybrid systems
No full textThe study of charge current fluctuations (noise) can give useful insights into the properties of nanoscale systems. In this work, the peculiar properties of noise in multiterminal hybrid normal-superconducting systems are explored in the thermal out-of-equilibrium regime, i.e., when temperature biases are present (-noise). Using the Landauer-Büttiker approach, we identify two contributions: background noise and excess noise, analyzing them when both electrical and thermal biases are applied. When temperature biases are present, and superconducting terminals are grounded, we find that the first contribution depends not only on the electrical conductance, as the Johnson-Nyquist at equilibrium, but also on a quantity strictly related to the heat conductance. This is our first main result. On the other hand, the second contribution shows, as expected, additional terms originating from the partitioning of currents into different transport channels, including the ones associated with Andreev reflection processes. However, noise induced by the temperature differences unveils also interference terms that cannot be present either in voltage bias or in the absence of any Andreev processes. Finally, we apply the results obtained to two different specific physical situations. The first is a generic three-terminal normal-superconductor-normal system and the second is a device based on spin-resolved copropagating chiral channels in the integer quantum Hall regime with a superconducting region. In these example setups, we investigate mainly the shot-noise regimes, when high-voltage or high-temperature biases are applied. We find remarkable differences between the two limits, which ultimately show the different nature of electrically and thermally induced charge current fluctuations
Finite-frequency noise in a topological superconducting wire
No full textIn this paper we study the finite-frequency current cross-correlations for a topological superconducting nanowire attached to two terminals at one of its ends. Using an analytic 1D model we show that the presence of a Majorana bound state yields vanishing cross-correlations for frequencies larger than twice the applied transport voltage, in contrast to what is found for a zero-energy ordinary Andreev bound state. Zero cross-correlations at high frequency have been confirmed using a more realistic tight-binding model for finite-width topological superconducting nanowires. Finite-temperature effects have also been investigated
Heat-charge separation in a hybrid superconducting quantum Hall setup
No full textSeparating heat from charge in a material is an extremely challenging task since they are transported by the very same carriers, i.e., electrons or holes. In this Letter we show that such separation can reach 100% efficiency in a hybrid superconducting quantum Hall setup, provided that the quantum Hall system is tuned to the integer filling factor. We present microscopic calculations for a three-terminal setup to illustrate our idea
Thermopower of three-terminal topological superconducting systems
No full textWe study the thermopower of a three-terminal setup composed of a quantum dot
attached to three electrodes, one of which is a topological superconductor. In
the model, superconductivity is explicitly taken into account. We compare the
results for s-wave (trivial) and p-wave (topological) superconductors and
observe that for small temperatures the thermopower has different sign in the
two cases. This behavior is strongly dependent on temperature and we estimate
an energy scale that controls the sign in the p-wave case, which results
proportional to the square root of the gap and the coupling to superconductor.
The analytical results obtained with a simple 1D model are confirmed by a more
realistic tight-binding model
Enhanced thermoelectricity in nanowires with inhomogeneous helical states
No full textSemiconductor nanowires (NWs) with strong Rashba spin-orbit coupling (RSOC), when exposed to a suitably applied Zeeman field, exhibit one-dimensional helical channels with a spin orientation locked to the propagation direction within the magnetic energy gap. Here, by adopting a scattering-matrix approach applied to a tight-binding model of the NW, we demonstrate that the thermoelectric (TE) properties can be widely controlled by tuning the misalignment angle phi between the spin-orbit directions of two NW segments. In particular, when the RSOC vectors are antiparallel (Dirac-paradox configuration), we predict a significant violation of the Wiedemann-Franz law, and a strong enhancement of the Seebeck coefficient and the ZT figure of merit. We also show that the Zeeman gap determines the optimal energy window for doping and temperatures. These results suggest that controlling the spin-orbit field direction, which can be achieved with suitably applied wrap gates, is a promising alternative for tuning and optimizing the TE response in quantum-coherent semiconducting NW devices
Nonlocal superconducting correlations in graphene in the quantum Hall regime
Full text linkWe study Andreev processes and nonlocal transport in a three-terminal graphene-superconductor hybrid system
under a quantizing perpendicular magnetic field [G.-H. Lee et al., Nat. Phys. 13, 693 (2017)]. We find that
the amplitude of the crossed Andreev reflection (CAR) processes crucially depends on the orientation of the
lattice. By employing Landauer-Büttiker scattering theory, we find that CAR is generally very small for a zigzag
edge, while for an armchair edge it can be larger than the normal transmission, thereby resulting in a negative
nonlocal resistance. In the case of an armchair edge and with a wide superconducting region (as compared to
the superconducting coherence length), CAR exhibits large oscillations as a function of the magnetic field due to
interference effects. This results in sign changes of the nonlocal resistance
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